A sclereid is a type of thick walled highly lignified cell found in many plants. Sclereids that are found in trees are called stone cells. Stone cells can be found in the cortex, phloem and pith of several species of either hardwood or softwood trees. They are most commonly found in the bark of the tree. When pulp and paper mills use tree species that contain stone cells and use the whole tree with no debarking, when debarking is not efficient (as in the winter), or when species are used which contain stone cells in the heart of the tree, the stone cells will appear in the finished product. This is true for both chemical and mechanical pulps. Furthermore, some stone cells remain in the final product even after additional measures are taken to remove them.
Stone cells that are present in the papermakers furnish often cause difficulties. For instance, in papermaking mills where high-speed paper machines are employed the stone cells may produce areas of weakness on the forming sheet, resulting in more frequent breakage thereof. Breaks on the paper result in down time and loss of production. Accordingly, the number of stone cells is a quality issue in pulp, and pulp is commonly sold with a maximum stone cell count specification.
Furthermore, stone cells cause problems for the end user. A stone cell on a calendared sheet of paper causes an opaque spot, often referred to as a fish eye, to form. Fish eyes are undesirable, as they shed ink and show up as flaws in the print.
Currently there is no universal method for determining and quantifying stone cells. The methods currently in use are time consuming and operator dependent. Furthermore, many mills have adopted their own, in house methods for determining stone cells.
One method relies upon the hardness of the stone cell as a way of identifying a stone cell within a fiber matrix. In a dark room a light source is placed at an angle to a non-calendared hand sheet that is made using either standard method TAPPI T 205 or CPPA C.4. Where a shadow appears, the bump causing the shadow is checked to see if the bump is solid, for instance by rubbing the bump with a pencil. A hard bump is counted as a stone cell. This method is very time consuming and is highly operator dependant.
Another method relies upon the tendency of stone cells to form fish eyes. The method involves making a standard hand sheet as described above, and then calendaring the sheet between two hardened steel rollers under several hundred pounds of hydraulic pressure. The hand sheet is then put onto a light box and the places where the paper has circular opaque spots are counted as stone cells. Unfortunately, the fish eyes can be very small and poor hand sheet preparation renders them difficult to see. This method also is highly operator dependant.
In yet another method, a sample of a bleached kraft pulp stock is stained, spread in a thin layer, and examined in transmitted light. Differences in color, size and opacity make the stone cells easer to identify. Unfortunately, both the pulp stock and the stone cells take up the stain, and as such the differences in colour and opacity may not be easily discernable using transmitted light. Further, a same stain may be useful for one pulp stock and less useful for another rendering the application of the method more complex than desired.
An instrument for determining stone cells has been developed by Optest Equipment Inc., which uses a hand sheet of a known weight, thickness, and diameter that is put over a source of light. The instrument includes a camera mounted above the sample to look for subtle colour differences in a magnified, and therefore small, portion of the hand sheet. Areas of the hand sheet having a colour different from that of the surrounding fiber matrix are counted as stone cells. Of course, the small sample size is a major drawback of this method. Furthermore, stone cells often are of a similar colour compared to the surrounding fiber matrix, making such a colour based determination of stone cells difficult and unreliable. It is a further disadvantage that the instrument must be calibrated prior to analysis and that all parameters must remain constant from sample to sample in order for the instrument to stay in calibration.
Thus, it has been a continuing problem to provide a method and apparatus for quantifying stone cell content of paper or pulp. In addition, there has been a long-standing, unfulfilled need for a universal method for quantifying stone cell content of paper or pulp that is at a same time rapid and substantially operator independent.